Studies of algorithms and related imaging techniques for industrial inspection

Abstract

This thesis will deal with algorithms and imaging techniques for use in automated industrial inspection. The work falls into two main areas, the first dealing with general problems relating to typical inspection tasks, the second with specific applications including the analysis of seals on plastic packets.The requirements of a general object location and inspection system will be discussed initially in relation to algorithms supplied with commercial systems, which often seem ad-hoc. This will be followed up with detailed analyses of several corner and small hole detection algorithms. The features looked for in a useful algorithm are: (1) a high execution speed when implemented on a general purpose microcomputer, (2) good accuracy in locating the desired features, (3) robustness when faced with poor quality, noisy or cluttered images and (4) the ability to distinguish between genuine features and others that appear, superficially, to be similar. A program using these feature detectors to locate partially occluded machine parts in typical images will be presented.The second main area of investigation is that of the detection of faults in heat sealed food packets and is one which has hitherto largely been overlooked. The main problem with these packets is that the cellophane wrapper is highly reflective, giving rise to large areas of glare in any off-camera image. Experience has shown that careful lighting arrangement alone will never totally remove this problem. However, a simple arrangement of switched light beams, along with computer processing, can almost totally eliminate the glare. This approach has been used in the inspection of packets where faults are revealed by parts of the product inside showing through holes in the wrapper. Alternatively, by careful alignment of the light sources, the surface structure of the sealed part of a packet may be revealed. This can reveal defects either through the absence of a regular pattern, or by the presence of wrinkles running across the seal. Algorithms have been developed demonstrating each of these inspection tasks.Overall the work presented in this thesis has spanned several traditional areas of interest, and has also developed the techniques required for packet inspection and other situations where glare is a problem.